1. Field of the Invention
The instant invention discloses a non-optical shear-force feedback method to regulate tip-sample distance for all types of scanning probe miscroscopy. The feedback signal is derived from an electrical impedance change in a dithering piezoelectric element with attached scanning tip.
2. Brief Description of the Prior Art
Scanning probe microscopy (SPM) is used to map the minute scale surface topography of samples using feedback techniques to control a measurement probe at a fixed, user defined separation distance from the surface of the sample. By moving the relative lateral position of the probe and sample, and monitoring the change in feedback signal required to maintain a fixed local separation distance, the physical topography of the sample surface can be determined with resolution between 0.01 and 100 nm. Scanning force microscopes (SFM) also have extensive applications in sub-micron photo-lithography, electrical characterization, tribology, and atomic manipulation. Because the feedback distance regulation mechanism must provide accurate movement on an atomic length scale, a SPM system must have a highly sensitive and well-characterized probe-to-sample separation detection element.
Non-optical control of the distance between a scanning microscope probe tip and the surface of a sample provides numerous advantages, without the complications arising from unwanted optical interference and the requirement of precise optical alignment. The technique involves sensing the effects of surface damping on a vibrating probe by measuring the change in the AC electrical impedance of an electromechanical assembly consisting of a piezoelectric element and an attached scanning probe tip (piezo/tip), as the resonantly oscillating tip is brought within .sup..about. 10 nm of a sample surface.
The Schering-style bridge has been used to balance out most of the piezo/tip impedance with the tip retracted. However, while the impedance of the piezo alone is almost purely capacitive, the impedance of the piezo/tip assembly at one of the tip's mechanical resonances acquires a significant real part. The frequency dependence of the piezo/tip impedance near a tip resonance does not correspond to any simple combination of resistors and capacitors. While this impedance can be balanced by a series RC at any particular resonant frequency, the frequency range and ultimate sensitivity of a passive RC bridge turns out to be inconveniently limited.
Two non-optical distance control methods have recently been developed specifically for near-field scanning optical microscopes (NSOM): (1) electron tunneling current and (2) detection of fiber tip dither amplitude with a quartz piezoelectric tuning fork. Vacuum electron tunneling is similar to scanning tunneling microscopy by utilizing the metal coating on the optical fiber as the tip electrode. Electron tunneling is established when the tip is brought very close to the surface (&lt;1 nm). At this distance, the small electric current that results when an electron moves between the surface of a metal and the metallic probe is detected, through a thin separating layer of vacuum or air. The current drops exponentially with the probe-to-sample separation. This offers high enough sensitivity to achieve atomic scale resolution, but is strictly limited to probe and samples that are made of electrically conducting materials. While the tunneling current is extremely sensitive to distance, this feedback method requires the sample to be conducting and cannot be used for non-conductive probes or samples. This requirement significantly limits the power of this feedback method as applied to NSOM, which is an optical microscope and thus can be used to probe optical properties of any material.
Piezoelectric and piezoresistive sensors are used in some forms of SFM and NSOM. These techniques work by measuring the change in either voltage or resistance of a sensor mounted to the probe assembly, usually set in vibration, as the probe is deflected by the sample. The sensor required is typically a highly specialized, tuned piezoelectric or piezoresistive element. Typically a tapered fiber is glued on a high Q (quality factor) quartz tuning fork with resonance frequency of 32,768 Hz. The tip-sample interaction will change the Q of the tuning fork plus tip assembly. Therefore, measurement of the piezoelectric signal from the tuning fork can serve as a signal for distance control. Although this method has been shown to work reasonably well, there remain problems. The attachment of a quartz tuning fork and wires near the fiber tip can obscure collection for reflection NSOM imaging. In addition, the Q of commonly available quartz tuning forks is high (-1000), resulting in limited bandwidth and the need for slow scan rates. Similar to capacitance sensors, these methods normally require attachment of an additional transducer with at least two extra wires dedicated to the sensor, in addition to the voltage driving wires on the mechanical oscillator. The chief drawback of these techniques is their need for specially fabricated driving or sensing elements and their relatively limited frequency range of operation.
Optical scattering, deflections, and interferometry are used in many forms of SFM and NSOM. This method involves using light scattering to detect the change in deflection of a probe as the probe-to-sample distance changes. However, optical detection is frequently not useful or convenient in many situations because it requires high precision mechanical alignment of light source, probe, and light detector. This makes optical detection difficult to use, and it is particularly unreliable for use in hostile or unusual environments, such as high vacuum chambers, corrosive atmospheres, or low-temperature cryostats. Scientific and industrial studies of surface properties are often carried out under such conditions, and so can benefit greatly from adapting SFM and NSOM to work reliability in such environments. In addition, stray light from the optical detection systems make it undesirable for use with light-sensitive samples, which is a particular drawback for use with NSOM.
Capacitance sensors are used in some forms of SFM. This method measures the change in capacitance on a capacitance sensor mounted on the probe as the probe-to-sample distance changes. Capacitance sensing suffers from more limited sensitivity and requires the attachment of a specialized external sensing element, the capacitance transducer, and electrical leads onto the probe assembly. This greatly increases the complexity and decreases the design flexibility of the probe assembly while offering less sensitivity than other methods.
NSOM is a novel technique that achieves optical resolution higher than the diffraction limit by placing a subwavelength aperture less than a wavelength from a sample surface, i.e. in the near-field regime. The most commonly used subwavelength apertures are fabricated by tapering, and then metal-coating, optical fibers. The images formed with optical contrast are obtained by moving the aperture laterally relative to the sample, similar to other scanning microscopy techniques. The high spatial resolution arises from the interaction of evanescent waves, which exist only near the aperture and which decay rapidly away from the aperture. Since the evanescent modes contribute only in the near-field regime and show strong distance dependence, the aperture-sample distance regulation is essential for high resolution imaging and for the interpretation of near-field optical contrast. Furthermore, the metal (usually aluminum) coating used to define the NSOM aperture is soft, therefore distance regulation for NSOM must prevent physical contact between the fiber tip and the sample in order to prevent damage to the aperture. Typical operation for visible and near infrared light uses aperture sizes of 20 to 300 nm and sample-to-aperture (i.e. fiber tip) separation of 1 to 20 nm.
A shear-force mechanism has been widely adapted to regulate fiber tip-sample separation in NSOM. The tapered optical fiber is attached to a piezoelectric element (the dither piezo) and held vertically above the sample surface. At the end of the fiber is the subwavelength aperture. The tapered fiber tips are mechanical structures with lateral vibration normal modes of reasonably high quality factor (Q.about.30 to 100), analogous to a long rod held at one end. By applying a time-dependent (AC) voltage to the dither piezo at a selected resonant frequency, the fiber tip is set in vibration parallel to the sample surface. As the tip approaches the sample, the amplitude of this dithering motion decreases due to interaction with the sample. In other words, the Q of the mechanical system decreases. The prior art shear-force feedback involves detecting the tip dither amplitude and regulating tip-sample separation by maintaining a fixed user-defined dither amplitude.
The first two methods for shear-force distance control in NSOM were independently proposed by Betzig et. al. and Toledo-Crow et. al. Both methods are based on optical detection and require use of a second laser and photodetector in addition to what is needed for the NSOM imaging itself. The widely used method works by focusing the feedback laser onto the fiber tip and monitoring the AC component of the scattered light at the dither frequency. Although this optical method regulates distance well, the major disadvantage is that while the NSOM light from the subwavelength aperture is -10 nW in power, the feedback laser power is typically 10.sup.5 times larger, roughly 1 mW. The optical excitations arising from the feedback laser can result in a large background signal and reduce the signal-to-noise (S/N) ratio for the near-field optical contrast. Through a careful selection of feedback laser wavelength and usage of proper filters, some of these problems can be minimized. However, to accomplish this, the feedback laser must then be sample specific. Further, optical feedback makes NSOM studies of narrow-bandgap semiconductors (bandgap.ltoreq.0.5 eV) and superconductors (superconducting gap.ltoreq.10 meV) impractical. In addition, crucial alignment of the feedback laser, fiber tip, and the photodetector makes it difficult to adapt optical detection methods for operation under non-ambient conditions, e.g. at low temperature or in ultra-high vacuum. Thus, a non-optical method for tip-sample distance control is desirable.
The instant invention discloses a probe-to-sample separation detection method based on monitoring changes in the electromechanical power dissipation of an oscillating probe as the probe-to-sample distance is changed. While maintaining the sensitivity of the prior art, the instant method provides significantly greater simplicity and a wider range of applicability.